Over the past several years, a number of factors have been shown to govern B cell immunopoeisis; I.E. the transition of naive B cells to GC B cells, memory B cells, isotype-switched B cells and B cells that have undergone somatic hypermutation. Unfortunately, there are no systems that can incisively evaluate each of these transitions because of the frequency of each of these events is so low. We present a system that will allow that exploration of these events at a high frequency. The novel features of the system that we present employs the use of 1) Ig-Tg B cells that using a site-directed rearrange VDJ inserted in the heavy chain locus with a well- defined specificity (nitrophenol, NP, known as quasi-monoclonal B cells QM); b) Tg CD4/+ T cells that can induce the differentiation of the QM B cells, c) the application of a series of reagents and knock-out mice that can selectively block the activities of factor shown to interfere with B cell immunopoeisis. The first specific aim is to define the characteristic changes that occur as QM B cells are driven to expand and differentiate in response to T cell help and a thymus-dependent antigen. For the first time, high frequencies of antigen-specific centroblasts, centrocytes, memory B cells and hyper- mutated B cells should be definable. It is anticipated that a more comprehensive understanding of the biology and regulation of each of these B cell subsets will be obtained. With this system in hand, the role of a series of relevant TNF family members in regulating discrete phases of B cell immunopoeisis will be studied. The impact of blocking factors known to be crucial to B cell immunopoeisis, like CD154, LTalpha, TNFalpha, OX40, and complement will be studied in detail. In parallel to studies of TD humoral immune responses in the QM system, development of the use of QM B cells to investigate studies of TD humoral immune responses in the QM system, development of the use of QM B cells to investigate thymus- independent (TI) immune responses will also be initiated. In contrast to TD responses, the response is to TI antigens is typified by low affinity anti-NP antibody. Conversion of the low affinity, non-anemnestic TI response to a high-affinity , anemnestic response will be achieve through the use of agonists that trigger TNFR family members (sCD154, anti-OX40, TNFalpha). Transitions from TI to TD-type responses will held resolve how each of these components influence B cell immunopoeisis.